Ballistic Background: Barrel

To understand how machine guns work, it helps to know something about firearms in general. Almost any gun is based on one simple concept: You apply explosive pressure behind a projectile to launch it down a barrel. The earliest, and simplest, application of this idea is the cannon.

Photo courtesy Department of Defense U.S. Marines fire a M-240G machine gun during training exercises at Camp Lejeune Marine Corps Base in North Carolina. Medium machine guns such as this one are an essential element in the modern arsenal. See more pictures of machine guns.

A cannon is just a metal tube with a closed end and an open end. The closed end has a small fuse hole. To load the cannon, you pour in gunpowder (a mixture of charcoal, sulfur and potassium nitrate), and then drop in a cannonball. The gunpowder and cannonball sit in the breech, the rear part of bore, which is the open space in the cannon. To prepare the gun for a shot, you run a fuse (a length of flammable material) through the hole, so it reaches down to the gunpowder. To fire the cannon, you light the fuse. The flame travels along the fuse, and finally reaches the gunpowder.

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When you ignite gunpowder, it burns rapidly, producing a lot of hot gas in the process. The hot gas applies much greater pressure on the powder side of the­ cannonball than the air in the atmosphere applies on the other side. This propels the cannonball out of the gun at high speed.

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